In a nutshell, short term memory problems. The ability to commit to memory is preceded by attention. The difficulty remembering everyday things like where you left something, appointments, assignments, due dates, work completion, what you ate for breakfast, etc. are a challenge for those with ADHD. Unlike the average brain, the ADHD brain is motivated by their search for optimal stimulation rather than what others label as important. The degree of arousal depends on whether the request is from internal desire vs. external demand. In the case of ADHD, internal desire leads to high arousal and external demands are often kicked to the wayside.